Evidence from clinical and experimental studies indicates that elevated intestinal permeability to endotoxins and the resulting endotoxemia play a crucial role in the pathogenesis of alcoholic liver disease. Our studies conducted so far have shown that acetaldehyde, the metabolic product of ethanol, disrupts the intestinal epithelial barrier function and increases the permeability to endotoxins. The mechanism of this acetaldehyde-induced disruption of epithelial barrier function involves inhibition of a protein tyrosine phosphatase, PTP1B, tyrosine phosphorylation of junctional proteins, disruption of the interactions among the junctional proteins (that determine the barrier function), and loss of integrity of the junctional complexes. Furthermore, our studies demonstrated that epidermal growth factor (EGF) and L-glutamine prevent acetaldehyde-mediated increase in permeability to endotoxins by a PLC?, PKC?, PKC?I and calcium-dependent mechanism. Our preliminary studies indicate that acetaldehyde induces translocation of PP2A leading to dephosphorylation of occludin and claudin-4, ethanol amplifies the effect of acetaldehyde by a Src kinase and MLCK-dependent mechanism and that probiotic, L. plantarum prevents acetaldehyde- induced barrier disruption. On the basis of these results it is further hypothesized that: a) ethanol metabolism and gut microflora play crucial roles in ethanol-induced intestinal barrier dysfunction, b) PP2A-dependent dephosphorylation of occludin and Cldn-4 is involved in acetaldehyde-induced disruption of intestinal barrier function, c) ethanol synergizes acetaldehyde-induced barrier disruption by c-Src-mediated MLCK activation, and d) probiotic, L. plantarum, prevents ethanol and acetaldehyde-induced disruption of barrier function by a EGF receptor, p38MAPK and Rac1- dependent mechanism. Using a cell culture model of the intestinal epithelium and human colonic biopsies we will determine that: 1) ADH1B and ALDH2 modulate ethanol-induced disruption of TJs and barrier function. 2) ALDH2 deficient mice are more sensitive to ethanol-induced barrier dysfunction. 3) Gut microflora play a role in ethanol metabolism and ethanol-induced disruption of TJs. 4) Acetaldehyde-induced PP2A methylation and translocation leads to dephosphorylation of TJ proteins and disruption of barrier function. 5) Dephosphorylation of occludin and Cldn-4 on specific Ser and Thr residues is associated with acetaldehyde-induced disruption of TJs and barrier dysfunction. 6) PP2A translocation plays a role in acetaldehyde-induced TJ disruption in mouse intestine. 7) Ethanol-mediated c-Src activation synergizes acetaldehyde-induced TJ disruption. 8) MLCK mediates synergization of acetaldehyde-induced TJ disruption by ethanol. 9) Ethanol sensitizes mouse colon for acetaldehyde-induced barrier dysfunction by a c-Src and MLCK- dependent mechanism. 10) L. plantarum prevents ethanol/acetaldehyde-induced disruption of junctions by p38MAPK-dependent mechanism. 11) Rac1 activation and stabilization of actomyosin ring are involved in the L. plantarum-mediated prevention of ethanol/acetaldehyde-induced tight junction disruption. 12) L. plantarum ameliorates ethanol/acetaldehyde-induced intestinal barrier dysfunction in mice and human colonic mucosa. The outcome of these studies has a direct relevance to our understanding of the pathogenesis of alcoholic liver and pancreatic diseases, and has the potential to contribute to the future development of new therapeutic strategies.

Public Health Relevance

On the basis of our research during the past several years we hypothesized that ethanol metabolism by gut microflora into acetaldehyde disrupts intestinal epithelial barrier function by inducing phosphorylation of proteins of intercellular junctions, and the probiotic L. plantarum prevents such cellular damage by acetaldehyde. We propose to conduct studies to uncover the cellular and molecular mechanisms involved in these processes and determine the protective role of a probiotic in alleviating the alcohol- induced tissue injury. The outcome of these studies is expected to provide knowledge to develop new therapies in the treatment of alcoholic liver disease and alcohol-induced tissue injury in pancreas and lung.

National Institute of Health (NIH)
National Institute on Alcohol Abuse and Alcoholism (NIAAA)
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Xenobiotic and Nutrient Disposition and Action Study Section (XNDA)
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Gao, Peter
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University of Tennessee Health Science Center
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Chaudhry, Kamaljit K; Shukla, Pradeep K; Mir, Hina et al. (2016) Glutamine supplementation attenuates ethanol-induced disruption of apical junctional complexes in colonic epithelium and ameliorates gut barrier dysfunction and fatty liver in mice. J Nutr Biochem 27:16-26
Shukla, Pradeep K; Chaudhry, Kamaljit K; Mir, Hina et al. (2016) Chronic ethanol feeding promotes azoxymethane and dextran sulfate sodium-induced colonic tumorigenesis potentially by enhancing mucosal inflammation. BMC Cancer 16:189
Mir, Hina; Meena, Avtar S; Chaudhry, Kamaljit K et al. (2016) Occludin deficiency promotes ethanol-induced disruption of colonic epithelial junctions, gut barrier dysfunction and liver damage in mice. Biochim Biophys Acta 1860:765-74
Chaudhry, Kamaljit K; Samak, Geetha; Shukla, Pradeep K et al. (2015) ALDH2 Deficiency Promotes Ethanol-Induced Gut Barrier Dysfunction and Fatty Liver in Mice. Alcohol Clin Exp Res 39:1465-75
Samak, Geetha; Chaudhry, Kamaljit K; Gangwar, Ruchika et al. (2015) Calcium/Ask1/MKK7/JNK2/c-Src signalling cascade mediates disruption of intestinal epithelial tight junctions by dextran sulfate sodium. Biochem J 465:503-15
Rao, Radhakrishna K (2014) Commentary: acetaldehyde and epithelial-to-mesenchymal transition in colon. Alcohol Clin Exp Res 38:309-11
Samak, Geetha; Gangwar, Ruchika; Crosby, Lynn M et al. (2014) Cyclic stretch disrupts apical junctional complexes in Caco-2 cell monolayers by a JNK-2-, c-Src-, and MLCK-dependent mechanism. Am J Physiol Gastrointest Liver Physiol 306:G947-58
Rao, Rk (2013) Sealing the ducts: Tight junctions of ductal epithelium. Tissue Barriers 1:e27772
Rao, R K; Samak, G (2013) Bile duct epithelial tight junctions and barrier function. Tissue Barriers 1:e25718
Rao, R K; Samak, G (2013) Protection and Restitution of Gut Barrier by Probiotics: Nutritional and Clinical Implications. Curr Nutr Food Sci 9:99-107

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